In order to interchange data between different communication terminals, an identical clock or oscillator signal is required for the transmission and reception of data in the subscriber appliances. When interchanging data via radio links, this is generally a radio-frequency clock or oscillator signal (for example at 2 GHz or more). This clock or oscillator signal must be produced in the various communication terminals independently of one another, for which reason the individual clock signals must be highly frequency stable, in order to maintain synchronicity between the subscriber appliances. Thus, in general, electromechanical oscillators, such as oscillating crystals or similar piezoelectric oscillators, are used to produce the stable-frequency clock or oscillator signal.
Crystal oscillators have a high power consumption, which is particularly disadvantageous in battery-powered terminals or terminals which are independent of the mains system, such as mobile telephones, since the comparatively high energy consumption shortens the operating time of such terminals which are independent of a mains system. The oscillating crystals which define the frequency are therefore not generally operated permanently but only at those times at which data is actually being interchanged, or when such an interchange is intended to be possible.
If the intention is just to ensure the capability to interchange data (for example during the standby mode of mobile telephones), then the subscriber appliance is generally briefly switched to reception at regular intervals, for which purpose the radio-frequency clock signal is required. In consequence, the crystal oscillator is likewise switched on for short periods at regular intervals. The radio-frequency clock signal is in this case generally required only for a few milliseconds. However, it must be remembered that the crystal oscillator itself also has an oscillation starting period of several milliseconds.
FIG. 1 illustrates the measured oscillation starting process of a known crystal oscillator. The supply voltage is switched on at t=0, and the oscillating crystal has stabilized after about 4 ms. The radio-frequency clock signal is therefore not available until several milliseconds after the oscillating crystal has been switched on. If the radio-frequency clock signal is likewise required only for a few milliseconds, then the oscillation starting process of the crystal oscillator is lengthened, during which process the radio-frequency clock signal, whose overall operating time is significant, is not yet available. The overall operating time and thus the energy consumption of the crystal oscillator when operated for short periods in this way are governed to a considerable extent by the duration of the oscillation starting process.